Fixation and electrical connection of an electronic component in electronic equipment, such as mounting of the electronic component on a printed board, are generally performed by soldering, which is most advantageous in terms of cost and reliability.
A method generally adopted for such soldering is a flow soldering method of bringing the printed board and the electronic component into contact with a molten solder to perform the soldering, and a reflow soldering method of remelting a solder having a form of a solder paste, a solder preform or a solder ball in a reflow furnace to perform the soldering.
In this soldering, a flux, which is an auxiliary agent for making solder easily adhere to the printed board and the electronic component, is used. The flux plays many useful actions such as: (1) a metal surface cleaning action (action of chemically removing oxide films on metal surfaces of printed board and electronic component to clean surfaces so that the surfaces can be soldered); (2) an reoxidation-preventing action (action of covering cleaned metal surface during soldering, blocking contact with oxygen, and preventing metal surface from being reoxidized by heating); and (3) an interfacial tension lowering action (action of reducing surface tension of molten solder and enhancing wettability of metal surface with solder).
In the soldering of the printed board by the flow soldering method, a flux (post flux) is applied to a portion to be soldered before or after the electronic component is mounted. After that, the printed board after the post flux has been applied is made to pass over the solder which is jetted, in the flow soldering apparatus, and flow soldering is performed.
As a flux used for soldering by a conventional flow soldering method, a no-clean resin-based flux for a lead-free solder which contains at least one compound selected from a base resin, an activator, an acid phosphate ester and a derivative thereof, in an amount of 0.2 to 4 mass % is proposed in PTL 1. In an Example of PTL 1, a flux which uses an amine borohydrofluoride as an activator, and a phosphonic acid ester (phosphonate) or a phosphoric acid ester (phosphate) as an acid phosphate ester is specifically shown.